Improve toString for flat classloader
Improve toString for flat classloader

It can be helpful to see what jars are available to the underlying url
classloader as well as what the parent classloader is.

xuwei-k/xsbt,mdedetrich/sbt,xuwei-k/xsbt,xuwei-k/xsbt,sbt/sbt,sbt/sbt,sbt/sbt,mdedetrich/sbt,sbt/sbt,mdedetrich/sbt,sbt/sbt,sbt/sbt,mdedetrich/sbt,mdedetrich/sbt,xuwei-k/xsbt,mdedetrich/sbt,xuwei-k/xsbt,mdedetrich/sbt,sbt/sbt